Question

In Thomson mass spectrograph when only electric field of strength 20 kV/m is applied, then the displacement of the beam on the screen is 2 cm. If length of plates = 5 cm, distance from centre of plate to the screen = 20 cm and velocity of ions = 10⁶m/s, then q/m of the ions is

• A. 10⁶C/kg
• B. 10⁷C/Kg
• C. 10⁸C/kg
• D. 10¹¹C/kg

Answer 1

Answer: (C)

10⁸C/kg

Answer 2

By using $y = \frac{qELD}{mv^{2}}$; where y = deflection on screen due to electric field only

⇒ $\frac{q}{m} = \frac{yv^{2}}{ELD} = \frac{2 \times 10^{- 2} \times (10^{6})^{2}}{20 \times 10^{3} \times 5 \times 10^{- 2} \times 0.2} = 10^{8}C/kg.$